Currently emergency vehicles operate a warning siren in order to warn people that the emergency vehicle is approaching so that those persons can take such action as is necessary to get out of the path of the emergency vehicle. Recent car design has improved sound deadening to isolate passengers from other noise. This makes it more difficult for drivers and/or passengers to hear a siren. Further, there is now wide spread use of sound systems in motor vehicles and a tendency for persons to listen to their sound systems at high volume levels which can make it difficult for them to hear a warning siren. Hence, audible warning devices used by emergency vehicles have become less effective.
U.S. Pat. No. 4,296,496 proposes a radio warning device for broadcasting a warning signal which can be received by radios in vehicles in the immediate area.
U.S. Pat. No. 5,889,475 proposes a system in which a radio signal transmitted by a moving vehicle is used to activate an AM/FM transmitter located at an intersection. Once activated, the transmitter broadcasts a warning signal which can be received by a vehicle's radio.
An inherent problem with such systems is that the warning signal will be received by radio receivers of people who do not need to receive the warning signal. While this cannot be avoided entirely, it would be advantageous to provide apparatus which aims to minimize the extent to which people who do not need to receive the warning signal are inconvenienced.